Free Living Flatworm Regeneration and the Practical Applications of Studying Their Regenerative Capabilities

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The ability to replace an injured portion of an organism's body is known as regeneration. Regeneration of free living flatworms is most common pronounce among "more basal taxa (Aceola, Catenulida, and Macrostomorpha) as well as in more derived taxa (Tricladida and Neodermata)" (Egger et al., 2007). The major free living flatworm taxa, with the exception of Nermodermatida, show some capacity to self regeneration damaged or missing tissues. The term planarian is used to designate a specific group of free living flatworms know as triclads. The triclads have been the focus of the majority of regenerative studies conducted on free living flatworms. The planarians are an ideal organism to study regarding regeneration because "they are one of the simplest bilaterians known to display robust regenerative capacities" (Alvarado, 2006). The planarians are mainly recognized "for their capacity to regenerate complete individuals from miniscule body parts" (Alvarado, 2006). Scientists have been particularly interested in flatworm regeneration recently because of its close relation to stem cells and embryogenesis. The mitotically active neoblast cells of the free living flatworms are currently being investigated in order to better understand the regeneration process. Somatic cells called neoblasts are found in the regeneration blastema of the flatworms. Neoblasts function by replaceing cells lost due to normal physiological turnover as well as replacing lost tissues due to amputation. The capability to regenerate is a trait that is expressed in very few of the animals in higher taxa than the free living flatworms. With such useful applications, researchers are looking into the reasons why this trait was not passed on to the higher animals. One of the main reasons proposed by researchers is that "other than as a side effect of asexual reproduction, the ability to regenerate is seemingly not useful enough to outweigh the inherent dangers" (Egger et al., 2007). A better understanding of the regeneration process in free living flatworms will provide for future advancements in regenerative medicine. This review aims to explain the factors that control and effect free living flatworm regeneration, and the practical applications of studying the regenerative capabilities of free living flatworms.

In order to best understand the processes of free living flatworm regeneration researchers are focusing on the factors that inhibit or incapacitate the regenerative capability of the flatworms. One of the earliest and most notable factors that is thought to have some effect on regenerative capability, is the ability of the organism to reproduce asexually. Egger notes that "in taxa lacking asexual reproduction, the regeneration capacity is generally less pronounced than in taxa with asexual reproduction" (Egger et al., 2007). There are two different forms of asexual reproduction that are exhibited in the free living flatworms. The first type of asexual reproduction, architomy, is where the "regeneration event follow fission" of the reproducing flatworm (Eggers et al., 2007). The second type of asexual reproduction in free living flatworms is called paritomy. Paritomy is where the organs are produced in the flatworm before fission. Paritomy is more accurately referred to as pregeneration due to the nature of the process. When fission during paritomy is artificially induced the daughter organism often fails to produce a head. The difference between architomy and paritomy could explain much of the difference between regenerative capabilities in flatworms that exhibit asexual reproduction. The main difference between regeneration and pregeneration is the presence of the head ganglion, "which is regarded as a decisive organ for regeneration in many species" of free living flatworms (Egger et al., 2007).

Based on several different experiments which dealt with _Paramecynostomum diversicolor_ and _Polychoerus caudatus_, researchers have determined that...
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